Copolymerization of exetanes (OX) with tetrahydrofuran (THF) above the ceiling temperature of THF leads to the formation of highly ordered copolymers if a large excess of THF (M2) is used. At [THF]/[OX] -∞ in the monomer feed, the alternating copolymer with a limiting composition [thf]/[ox] - 1:1 results if OX is 3,3-dimethyloxetane (DMO) or 3,3-bis(fluoromethyl)oxetane (BFMO). In a copolymeriza-tion with 3,3-bis(chloromethyl)oxetane (BCMO) the limiting copolymer composition is close to 2:1. These data suggest the influence of the penultimate unit in the latter pair due to the presence of the bulky chloromethyl groups. This influence is of electronic rather than of steric character since no difference in the structure of copolymers was observed for DMO and BFMO. Analysis of the Stuart molecular models reveals that the chloromethyl groups of BCMO adjacent to the growing center,, are frozen and cannot rotate freely. Thus the depropagation step of a ∼m1m2m2sequence, involving the back bending within, proceeds with a much more negative entropy of activation then the depropagation step of the similar sequence involving DMO or BFMO. Therefore the depropagation of the former is slower than that of the latter. The simplified kinetic scheme, applied for a copolymerization above the Tc of THF, allowed the reactivity ratios to be determined.